A resonance circuit tag having a circuit which resonates with a certain wave frequency has been used for preventing shoplifting in libraries and diverse kinds of stores such as those selling books, CDs, video tapes and the like, supermarkets and department stores. Various attempts have been made to provide an economical, compact and high quality resonance tag, as reported in Japanese Utility Model Unexamined Publication Nos. 54038/1973 and 75585/1973, and Japanese Patent Unexamined Publication Nos. 23395/1989, 130490/1987 and 200297/1988.
A resonance circuit tag is required not to function after the price of an article has been paid at a check-out counter, and to this end, the tag is usually removed from the article at the check-out counter. However, recent demand for a labor-saving and speedy operation of cash registers as achieved by the introduction of bar codes, etc. cannot be fully met due to the troublesome step of removing a resonance circuit tag attached to the article.
To overcome such disadvantage, a so-called self-breakdown type resonance circuit tag has been proposed which loses its resonance function by the application of heat and other methods. Specific examples thereof include a resonance circuit tag wherein a soluble link is formed in its circuit, which melts upon application of energy having a self-breakdown frequency, thereby breaking the circuit (U.S. Pat. No. 3,913,219) and a resonance circuit tag prepared from an electrically insulating sheet composed of a thermoplastic resin, etc. wherein a high frequency current is generated by applying a strong high frequency wave having a resonance frequency, by which said insulating sheet is deformed or melted to cause defective insulation, resulting in the functional termination of the resonance circuit (Japanese Patent Unexamined Publication No. 266700/1987).
In the self-breakdown tag of the type mentioned above, the the resonance function can be terminated by, for example, passing the tag on an article through a high frequency inductor; thus, and the aforementioned labor-saving, speedy operation of cash registers are not impaired.
Of the above-exemplified resonance circuit tags, the former is associated with a problem in that a large-scale production is difficult to achieve by a conventional circuit-forming technique such as etching, since part of the circuit needs to be formed more narrowly than the remainder.
On the other hand, the latter resonance circuit tag has a risk of adversely affecting the function of an article to which the tag has been attached, as a result of the considerably strong high frequency wave having a resonance frequency which is applied to the resonance circuit to deform or melt the insulating sheet composed of a thermoplastic resin. It may also suffer from insufficient self-breakdown function due to sufficient deformation of the insulating sheet.